Introduction

Sodium plays a key role in the study of metabolism and can provide relevant information on cell integrity in particular in the human brain. Several studies have taken advantages of the recent developments in scanner technology (ultra high field magnets, fast and strong gradients) and sequences design (ultra short echo time) to apply sodium MRI in patients in order to assess non-invasively total sodium accumulations in neurological disorders and diseases [1]. To obtain reliable quantitative sodium concentrations, accurate measurements of the bi-exponential ²³Na transverse relaxation times (T2*) are required. Some studies have reported the T2* short (0.5-5ms) and T2* long (15-30ms) sodium components of the human brain using only few echoes [1-2]. The objective of this study was to improve the assessment of T2* short and T2* long sodium components of the human brain at 7T using a dedicated multi-echo radial sequence.

Methods

All experiments were performed on a Siemens 7T Magnetom using a dual tuned ²³Na/¹H QED birdcage coil. A multi-echo density adapted 3D projection reconstruction ²³Na MRI sequence was designed (TR=120 ms, 10.000 spokes, 3.5mm³ isotropic resolution, 8 echoes, acquisition time = 20 min). This sequence was applied three times in order to obtain 24 TE ranging from 0.3 ms to 100 ms (0.3 – 0.8 – 2.3 – 6.3 – 7 – 9.7 – 12 – 13 – 15 – 17 – 19 – 21 – 23 – 25 – 28 – 31 – 35 – 40 – 45 – 54 – 66 – 80 – 89 – 100 ms). A high-resolution proton MRI 3D-MP2RAGE (TR=5000ms / TE=3ms / TI1=900ms / TI2=2750ms, 256 slices, 0.6mm³ isotropic resolution, acquisition time = 10 min) was obtained using a 32-element 1H head coil (Siemens). This protocol was applied in 14 healthy subjects (9M/5F, median age of 22 yo [range: 20-32]). For each echo time, sodium images were reconstructed using a gridding method [3] and after sodium images from all echoes times were realigned to sodium images from the first echo time (example showed in Figure 1). Proton images were segmented to white matter (WM) and grey matter (GM) compartment and the resulting maps of WM and GM were realigned and applied to the sodium images of the different echoes. A bi-exponential fitting was performed using Matlab in order to assess the short and long T2* component of the sodium signal decay according to the TE (Figure 2) for each subject.

Results

As illustrated in Figure 3, the sodium signal decreased rapidly according to the TE. The distribution of the T2* short and T2* long sodium values among all GM and WM voxels are displayed in Figure 4. The mean T2* short sodium in the GM (5.02 ± 0.89 ms) was significantly higher than in WM (4.50 ± 0.65 ms). The mean T2* long sodium in the GM (33.89 ± 5.96 ms) was significantly lower than in WM (39.99 ± 5.24 ms). The ratio between the sodium signal from the short component (M0short) and the long component (M0long) was 60% (M₀short) vs 40% (M₀long) in WM while it was 45% (M₀short) vs 55% (M₀long) in GM.

Discussion

This study using a dedicated multi-echo radial sequence allowed us to report values of the short and long components of the bi-exponential transverse decay of sodium in the human brain at 7T. Differences found between GM and WM are probably due to the different sub-structures involved. More subjects are needed to confirm these preliminary results in order to improve future sodium quantification studies at 7T.

Acknowledgements

No acknowledgement found.